### In a nutshell↵
`#define _GLIBCXX_SANITIZE_VECTOR 1`↵
↵
### Prerequisites↵
Many of you know about _GLIBCXX_DEBUG / _GLIBCXX_DEBUG_PEDANTIC defines that force libstdc++ on GCC compilers to check out-of-bounds access to the array and react correspondingly.↵
Linux (and some Mac OS users that use clang as a compiler) may also know about libasan library (that you can activate via providing `-fsanitize=address` parameter to the build option. It checks not only for STL containers, but also every possible out-of-bounds memory access. While it may seem like it's going to work on `std::vector` out-of-bounds access, that's not always the case. See the example below.↵
↵
~~~~~↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
↵
This will trigger the libasan check, as the allocated capacity of the vector equals to the size of the vector. More on how `std::vector` works you may read [here](https://en.cppreference.com/w/cpp/container/vector/push_back).↵
↵
**In a nutshell**: `std::vector` allocated some `capacity` that is >= than `size`, and after each `push_back` if `capacity` becomes lower than `size` it reallocates everything and **doubles** the capacity. ↵
#### So what's the problem? Why doesn't it work in all of the cases?↵
You see, when `size` < `capacity`, the allocated memory length is actually the size of `capacity`. ↵
↵
Imagine a situation: `size` equals to 9, `capacity` equals to 16. You access `arr[9]` and that won't go out-of-bounds from the point of an OS. Libasan won't trigger. You can test this code to prove that:↵
↵
~~~~~↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
arr.reserve(10000);↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
## What you have come for↵
If you pass `#define _GLIBCXX_SANITIZE_VECTOR 1` before including any libraries, `libasan` will trigger on container overflow in all of the cases. You can try the above example and see that it will work. So, the edited code will be:↵
↵
~~~~~↵
#define _GLIBCXX_SANITIZE_VECTOR 1↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
arr.reserve(10000);↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
### Motivation↵
I haven't seen any mentions of this on codeforces yet, so may be that would be helpful to someone.↵
↵
↵
`#define _GLIBCXX_SANITIZE_VECTOR 1`↵
↵
### Prerequisites↵
Many of you know about _GLIBCXX_DEBUG / _GLIBCXX_DEBUG_PEDANTIC defines that force libstdc++ on GCC compilers to check out-of-bounds access to the array and react correspondingly.↵
Linux (and some Mac OS users that use clang as a compiler) may also know about libasan library (that you can activate via providing `-fsanitize=address` parameter to the build option. It checks not only for STL containers, but also every possible out-of-bounds memory access. While it may seem like it's going to work on `std::vector` out-of-bounds access, that's not always the case. See the example below.↵
↵
~~~~~↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
↵
This will trigger the libasan check, as the allocated capacity of the vector equals to the size of the vector. More on how `std::vector` works you may read [here](https://en.cppreference.com/w/cpp/container/vector/push_back).↵
↵
**In a nutshell**: `std::vector` allocated some `capacity` that is >= than `size`, and after each `push_back` if `capacity` becomes lower than `size` it reallocates everything and **doubles** the capacity. ↵
#### So what's the problem? Why doesn't it work in all of the cases?↵
You see, when `size` < `capacity`, the allocated memory length is actually the size of `capacity`. ↵
↵
Imagine a situation: `size` equals to 9, `capacity` equals to 16. You access `arr[9]` and that won't go out-of-bounds from the point of an OS. Libasan won't trigger. You can test this code to prove that:↵
↵
~~~~~↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
arr.reserve(10000);↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
## What you have come for↵
If you pass `#define _GLIBCXX_SANITIZE_VECTOR 1` before including any libraries, `libasan` will trigger on container overflow in all of the cases. You can try the above example and see that it will work. So, the edited code will be:↵
↵
~~~~~↵
#define _GLIBCXX_SANITIZE_VECTOR 1↵
#include <vector>↵
↵
int main() {↵
std::vector<int> arr = {1, 2, 3, 4};↵
arr.reserve(10000);↵
int variable = arr[4];↵
}↵
~~~~~↵
↵
### Motivation↵
I haven't seen any mentions of this on codeforces yet, so may be that would be helpful to someone.↵
↵
↵
